Updated Plugin docs (markdown)

Plugin-docs.md

@@ -43,6 +43,21 @@ There's a handful of keys that can be used to achieve navigation within WebSlide
 * `Space`: Go to the next slide.
 * `Home`: Go to the first slide.
 * `End`: Go to the last slide.
+* `-`: Enter slide index.
+* `+` or `Esc`: Exit slide index.
+* `f`: Toggle fullscreen mode.
+
+## Slide Index
+
+The slide index is accesible by either pressing the `-` (dash) key or clicking on the slides count that's on the bottom of the page. It's accessible on mobile by pinching the screen.
+
+You can exit it manually by either pressing `+` or the `Esc` key and it also closes whenever you choose one of the slides to navigate to.
+
+If you'd rather like to disable this feature entirely, you can do so with the following option:
+
+```javascript
+const ws = new WebSlides({ showIndex: true });
+```
 
 ## Video